Full-wave silicon junction diode A.C. rectifying power packs are constructed of four basic elements -- diodes, electrical conductors for supplying power to and taking power from the diodes, heat sinks for transferring heat from the diodes and structure for supporting the diodes and the electrical conductors thereto. The problem for the designer of such equipment is to incorporate into a design concept the optimum geometric configuration of all these elements that gives the most economic and compact assembly of parts having an acceptable service life without prohibitive burdens of maintenance and repair.
Successful application of diode rectifiers depends to a great extent on adequate cooling. If the junction temperature rises high enough, permanent damage may occur in its characteristics and the device may fail by melting and thermal runaway. Circuits may fail before melting or thermal runaway occurs since insufficient cooling can reduce breakover voltage, and increase diode turn-off time, moving these and other diode characteristics outside specifications sufficiently to induce circuit malfunction. For all these reasons, all diode rectifiers are designed with some type of cooling fin or heat sink to dissipate internal heat losses.
The most common means of cooling stud-mounted diodes is to mount them directly to these cooling fins. Heat losses at the junction of the semiconductor will then flow down through the stud into the fin and then be dissipated to the ambient air by radiation and either free or forced convection heat transfer. The mechanism of heat transfer by forced air convection depends upon local turbulence in the air, thermal conduction down the electrical leads and the mounting for the fin, nearby radiant heat sources, and chimney cooling effects caused by other heated devices above or below the cooling fins. The final measure of the effectiveness of the cooling fin will always be the stud temperature which should never be allowed to exceed the manufacturer's rating for a given load condition.
For silicon junction rectifier diodes having a given heat dissipation rate, there is a lower limit on size for stacked fin assemblies. As fin spacing is reduced, shielding effects become more marked and radiation heat transfer is reduced so the manufacturer's stud temperature limitation may be exceeded.
For fins of thin material, the temperature of the fin decreases as the distance from the heat source or diode increases due to effects of surface cooling. The hottest spot is adjacent to the stud of the diode. The effectiveness depends on the length, thickness and shape of the fin. In general, fin thickness should vary approximately as the square of the fin length in order to maintain constant fin effectiveness. Also, a multi-finned assembly will generally have superior fin effectiveness and will make better use of material and weight than a single flat fin.
The service life of commercially available silicon junction diode rectifying equipment is mostly limited by the exposure of the diode for long periods of time to high operating temperatures from heat generated by the diode itself. The design objective is to dissipate this heat in the most efficient and economic way possible. It is the usual practice for such power converting equipment as silicon junction diode rectifiers to use forced circulation of ambient air for cooling purposes. The most efficient and often the most economical use of forced air convection for cooling of silicon junction diode rectifiers is that imposed on this equipment at minimum pressure drop across the equipment. Electrical circuitry, structural configuration of the heat sinks and structural support configuration are then prime factors affecting the development of pressure drop across the equipment and the distribution of cooling air between the elements therein. It is a present practice of the industry to transfer A.C. negative input currents from the end taps on the secondary winding of the main power transformer to the silicon junction diodes and D.C. positive output current from the diodes to the D.C. output bus bar through an extensive wiring system of multiple cable and lugit connectors. D.C. negative output current from the center tap of the main power transformer to the D.C. negative output bus bar is also handled in like manner. Multiple cables and light connectors increase cooling air pressure drop across the pack for a given cooling requirement.
It is also a present practice of the industry to have heat sinks carry positive D.C. current as well as conduct heat from the heat-generating silicon junction diodes themselves. Heat sinks each serve several silicon diodes and are not insulated from each other so that there is an intertransfer of heat, short circuits and conducting current from one cooling plate to another. D.C. current is also transferred from heat sinks to the main rectifier positive bus bar feeder. The use of heat sinks to conduct D.C. current as well as transfer heat increases the temperature of the heat sinks and the silicon diodes mounted thereon causing the diodes to conduct at considerably higher temperature.
Aluminum for both heat sinks and electrical components of silicon junction diode rectifier power packs has been much used through the industry, again because of cost effectiveness. With aluminum electrical conductors, contact resistance at the terminals generates heat and creates hot spots in the structure especially in the vicinity of the mounting of the silicon junction diode, again contributing to the early breakdown of the same.
There is thus an established need for a silicon junction diode rectifier power pack which gets more cooling benefit out of a forced air convection dollar in a smaller unit for an extended, maintenance-free service life.